Pressure within the separator is controlled at the required level in order to maintain optimum separation conditions and to provide the necessary pressure to send gas to the compressors and discharge the liquid to the down stream process.
A constant pressure is maintained by means of a pressure controller (PRC) which controls the opening of a PCV (usually pneumatically operated) in the gas outlet line.
The inlet pressure to the separator is manually controlled by the wellhead choke and inlet manifold choke. In some case gas from the separator will be directly routed to the gas compressors. Separator pressure will be maintained by the suction pressure controller of the compressor. The vessel pressure then floats with the gas compressor suction pressure controller and any extra pressure relieving to flare via pressure controller.
The separator is protected against very high pressure by means of pressure switches. Both high and low pressure switches will when activated, operate alarms and or shut the separator off from the flow station header. High pressure is relieved through a pressure relief system actuated by the high pressure switch.
Figure- Separator Safeguarding system
Level in the vessel is controlled by the LCV ( level control valve ) on liquid outlet line. In the three phase separators there will be two LCV`s to control the level of oil and water separately. By maintaining a level in the separator we are sure that enough time is given for the gas and oil to separate. The level control valve in the water outlet line, maintains a water level in the separator. This ensures that the water is retained in the vessel for sufficient time to allow oil to separate from the water
To monitor the process inside the vessel, the following gauges will be installed. Pressure gauge to indicate the operating pressure of the separator. Temperature gauge to indicate the gas phase operating temperature, Level gauges to show the operating liquid level in the separator.
Separator pressure, level , temperature and the flow of oil, gas and water can be monitored from the control room through SCADA system or through DCS. Totalised flow of gas and oil also indicated in the control room.
The vessel is protected by ESD valves on the feed inlet line and on the liquid outlet line. The main protection elements are:
Hiigh High Level ( LHH )switch is provide to detect a high oil level in the separator and it activates a process shutdown of the separator. This is to stop any liquid going out with the gas to the compressor or to the flare and to avoid overpressurising of the vessel
Low Low Level ( LLL ) switch to detect a low oil level in the separator and activate a process shutdown of the separator. This is to avoid gas going out through the liquid outlet line.
Level switch detect a low water level in the separator and it activates a process shutdown of the separator. This ensures that gas does not exit via the oil outlet.
Pressure switch (PHH)detects a high pressure in the separator and it activates a process shutdown of the separator. This ensures that vessel is not getting overpressurised.
7.4 Safety Relief Valves And Rupture Discs
In all production facilities, three levels of safety are provided. and these are
1. alarm system
2. shutdown system
3. safety relief system.
The majority of pressure relieving devices used in production operations are safety relief valves. These are automatic valves used to prevent excessive over pressure in vessels and piping systems.
The alarm and shut down systems are provided to allow an operator to detect and take corrective action in the event of an upset. The safety relief system must be provided to protect the vessel in the event the corrective reaction or shutdown is not made rapidly enough or in the event the alarm and shutdown systems fail.
Consider a high pressure production separator with a design working pressure of 1400 psi and which is operating at 1 000 psi.
Suppose something goes wrong and causes blocking of the outlet of the vessel, so that the pressure begins to build up inside the vessel.
First, provision is made to warn the operator that something abnormal is starting to occur. As the pressure builds up above the normal operating range of 1000 psi, say, to 1100 psi, and the operator may get a high pressure alarm
( warning ) This calls the operator’s attention that pressure is increasing, so that he can take corrective measures. Sometimes the alarm may not work or the operator was not in the control room, then the pressure will start increase again.
At 1200 psi, a second signal High High pressure switch comes in to action and a shitdown signal is sent to the separator and to close the inlet ESD valve. This should stop the pressure increase and protect the vessel from high pressure. But sometimes ESD valves don’t operate quickly enough to prevent over pressuring the vessel or don’t operate at all.
Then pressure continues to increase in the vessel, the third level comes into action. At a predetermined set pressure at or below the 1400 psi , the safety relief valve will open to relieve the pressure and protect the vessel. When the other protective devices fail to prevent over pressure or fail to operate, so the safety relief valve protects the vessel from rupturing by relieving the excess pressure.
7.5 Rupture Disc
Rupture discs are another pressure relieving device. Under certain conditions, rupture discs can be a very important element in a pressure relieving system.
Rupture discs are designed to rupture or burst automatically at a predetermined pressure. This is usually higher than the safety relief valve set pressure. The rupture disc, as a secondary measure, can be relied on to provide emergency over pressure protection.
A rupture disc opens by rupturing the thin metal disc held between the two flanges. It cannot be reset once operated, it must be replaced. They may be installed in parallel or in series with safety valves, or both. Rupture discs are often installed in series or immediately below a safety relief valve when it is necessary to protect the relief valve from continuous exposure to corrosive material. They should be installed in parallel or on a separate outlet, only if the loss of the system contents can be tolerated.